Field of the Invention
The present invention pertains to apparatus for producing finished dental restorations at the dentist's chair side from 3D CAD data. These restorations include but are not limited to, crowns, bridges, abutments, veneers and surgical guides. These may be manufactured from ceramics, metals and polymers via subtractive means i.e. milling and grinding. This new system will deliver many advantages over currently available chair side systems including; reduced margin chipping, increased accuracy, improved reliability and reduced complexity in a smaller package.
Background Art
The polar configuration of one prior art chair side system creates a compact mechanism suitable for chair side usage but represents a compromise from a programming stand point since the system must be programmed using polar coordinates. Toolpath generation algorithms are not well adapted to this purpose and polar movements are restrictive in terms of the machining strategy that can be employed. Another prior art chair side machine has a Cartesian mechanism which allows simple flexible toolpath programming but results in a much larger and heavier mechanism that is at odds with the intended chair side environment.
Both of these prior art systems utilize a toolpath that employs a lacing motion which crosses the margin with each pass. This places an extremely high acceleration demand on the machine which results in corner rounding and also a discontinuity in cutting force, both of which compromise the integrity of the margin.
By contrast, the present invention utilizes a predominantly helical motion that traces the actual form of the margin line. This significantly reduces the acceleration demands of the programmed path and maintains a constant grinding pressure. This results in a dramatic improvement in the marginal fit and integrity of the finished crown. This type of motion is not well suited to the aforementioned prior art machines. When using two tools simultaneously, it would be very difficult to avoid collisions and the necessary avoidance moves would cause rapid discontinuities in the machine's motion that are themselves likely to lead to damage if occurring close to the crown's margin.